Guessing Rules

When I tell my wife an incredible fact I expect her to be amazed. Normally I ask her to guess the answer before I tell her, because that way it forces her to think of what the 'highest' or 'biggest' could be before I tell her the astonishing truth. However, I have very strict but simple guessing rules that she must abide by for my ego to be satisfied:
Rule 1) Always guess 'less than' you think the real answer may be.
Rule 2) Don't guess so much 'less than' that it is obvious you are guessing 'less than'.
For example...

GUESSING TOO HIGH
Me: "What is the largest waist measurement of a human?"
The Wife: "Ummmmm, 15 foot?"
Me: "No! You didn't follow the guessing rules. It's 9 feet 11 inches!"

GUESSING TOO LOW
Me: "What is the wingspan of the biggest butterfly?"
The Wife: "Ummmmm, 2 inches?"
Me: "No! You didn't follow the guessing rules. It's 11 inches!"

GUESSING JUST RIGHT
Me: "How long are the world's longest toenails?"
The Wife: "Ummmmm, 3 foot?"
Me: "It's 7 foot, 3 inches!"
The Wife: "Wow! That's remarkable!"
Me: "I know. Do you want a foot rub?"